In lens technology, a differentiation is made between two areas of application and two categories of lens: optics, for which lenses made of mineral glass with spherical surfaces are usually used, and spectacles, for which lenses made of synthetic material with aspherical and non-rotationally symmetrical surfaces are usually used. The latter surfaces are polished using a polishing head with a zonally effective polishing method due to the lack of rotational symmetry, by contrast to which the spherical surfaces of the mineral glass lenses are polished all over with a polishing tool which has the required sphere. Additionally, it is provided that the spherical surfaces of the mineral glass lenses are polished zonally for the purpose of correcting the overall polishing procedure. For this purpose, a polishing head with a flexible polishing surface is usually used which serves to polish local ridges.
A method for polishing an aspherical, rotationally symmetrical surface of a lens by means of a tool which rotates around a tool axis is already known from DE 10 2004 047 563 A1. The workpiece is contacted in an area of a workpiece surface by an area which contacts it momentarily in each case, which is at least one partial area of an area to be machined, which is itself a partial area of a polishing area of the tool, wherein the tool axis penetrates a polishing area and the position of the tool is set in such a manner that the centre of the area of the tool which contacts the workpiece momentarily in each case (the polishing point) lies to the side of the tool axis. Here, it is provided that a tool with an even polishing area is tipped depending on a surface perpendicular of the workpiece in the area which is contacted, around an axis which differs from the tool axis, wherein the tool axis is aligned parallel to the surface perpendicular, and the tool is displaced parallel a workpiece surface in the area which is contacted. In the outer areas of the workpiece, i.e. outside the centre, the polishing capacity is controlled via the rotational speed. A variation of the radius of the polishing point on the tool side is not provided when machining the outer areas of the workpiece.
With reference to a section plane S which comprises the polishing point, the polishing point and the two section points SW and SL lie between the tool axis and the section plane S and between the workpiece axis and the section plane S on a straight line. Accordingly, the polishing point lies in the plane E which is tensioned by the tool axis and the workpiece axis.
A polishing device is known from EP 1 384 553 A2. This comprises a polishing head with a rotational axis, a pivot axis which is arranged at right angles to it, and a further pivot axis which is arranged vertically to both aforementioned axes. The polishing head can thus be rotated around all three spatial axes. On the workpiece side, a rotational axis is also provided. The polishing point, which can be generated, can thus be placed on the tool side decentrally to the rotational axis of the polishing head.
With reference to a section plane S which comprises the polishing point, the polishing point and the two section points SW and SL are in contact between the tool axis and the section plane S, and between the workpiece axis and the section plane S, or on a connecting straight line G, i.e. the straight line G passes through the polishing point. The polishing point comprises no distance a to the straight line G. When the tool axis and the workpiece axis tension a plane E, the polishing point lies in the centre on this plane E, or centrally to the straight line G. In this case, the polishing point also comprises no distance a to the plane E or to the straight line G.